the 3 months is there for two reasons from employers perspective
1) gives them time to find a replacement
2) makes it more difficult for you to find another job.
From yours, obviously it means that yu get 3 months to find another job if they want to get shot of you.
My experience is that unless they feel you’re going to a direct competitor and it’ll give the new emplyer some huge advantage to get you 2 months earlier it’s down to negotation as to when you leave and they’ll be flexible.
Also, unless they can prove they’ve lost out you can pretty much leave when you waqnt anyway – they’re unlikely to take you to court. You’d never work there again, though 😉